Polarization beam separator and combiner

ABSTRACT

The present invention relates to a device for use in a fiber optic system that may be a communication system, a sensing system or other system using guided-wave optical components. Reducing the number of lenses required to couple the waveguides and the free-space paths in the device offers the dual advantages of a reduced component count and simplified alignment. In an exemplary device having a first and second waveguides, a birefringent optical system defines bi-directional, polarization-dependent free-space paths. One of the bi-directional, polarization-dependent, free-space paths couples at least the first waveguide to the second waveguide. The birefringent optical system includes at least one prism for bending one of the polarization-dependent paths in a clockwise direction and one of the polarization-dependent paths in a counterclockwise direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/010,815, filed on Nov. 13, 2001, which issued as U.S. Pat.No. 6,741,764, and is incorporated by reference.

FIELD OF THE INVENTION

The present invention is directed generally to a fiber optic device, andmore particularly to devices that can split or combine light signalsaccording to the polarization of the light signals.

BACKGROUND

Optical fibers find many uses for directing beams of light between twopoints. Optical fibers have been developed to have low loss, lowdispersion, polarization maintaining properties and can also act asamplifiers. As a result, optical fiber systems find widespread use, forexample in optical communication applications and remote sensing.

Wavelength, optical power and polarization are important properties ofthe light signals propagating in a fiber optic system. Components withinthe system may modify the propagation of the signals by changing one ormore of these properties. For example, multiple signals may betransmitted through a single fiber optic by combining the outputs from aplurality of laser transmitters, each transmitter having an outputwavelength that is restricted to a unique spectral band. Amplitudeand/or frequency modulation may be used to encode information on thetransmitter outputs. The polarization property may be used for networkoperations that include the tuning, multiplexing, demultiplexing andswitching of light signals, for example.

Systems that utilize the polarization property of light often requirelight signals to be separated or combined according to theirpolarization state. A single fiber optic device may be designed to carryout both processes, separating signals from a combined input thatpropagates through the device in a first direction and combiningpolarized signals that propagate through the device in the oppositedirection.

Polarization beam separator/combiners for use in fiber optic systems mayuse non-guiding optical components to separate/combine the opticalsignals as they propagate through the device along free-space opticalpaths. Collimating lenses are typically used to couple the lightpropagating along the free-space optical paths to the input/outputwaveguides with a one-to-one correspondence between lenses andwaveguides. Thus a polarization separator/combiner with threeinput/output waveguides typically incorporates three lenses that must beaccurately aligned with respect to the waveguides and the free-spaceoptical paths.

Conventional polarization separator/combiners share several commondisadvantages that derive from the one-to-one correspondence betweenfibers and focusing optical systems. For example, the low-losspropagation of light is facilitated by the accurate alignment of theoptical focusing assemblies to the optical fibers. Alignment tolerancesmay be of the order of one micron and must be maintained against bothtemperature variations and vibration during the operational lifetime ofthe device. Typically, the optical components are housed in a mechanicalalignment and support assembly that increases in complexity, size andcost with the number optical coupling components. It is, therefore,disadvantageous to use a dedicated optical focusing assembly to coupleeach of the optical fibers

SUMMARY OF THE INVENTION

Generally, the present invention relates to a device for use in a fiberoptic system that may be a communication system, a sensing system orother system using guided-wave optical components.

Reducing the number of lenses required to couple the waveguides and thefree-space paths offers the dual advantages of a reduced component countand simplified alignment. It is, therefore, advantageous to provide apolarization splitter/combiner incorporating non-guiding opticalcomponents that interact with light propagating along free space paths,the free space paths coupled to a number, N, of input/output waveguidesby a number, M, of focusing elements where M<N.

One embodiment of the invention is directed to an optical device thatincludes a first waveguide, a second waveguide, and a birefringentoptical system with bi-directional, polarization-dependent free-spacepaths. One of the bi-directional, polarization-dependent, free-spacepaths couples at least the first waveguide to the second waveguide, thebirefringent optical system including at least one prism for bending oneof the polarization-dependent paths in a clockwise direction and one ofthe polarization-dependent paths in a counterclockwise direction.

Another embodiment of the invention is directed to an optical devicethat includes a first waveguide, at least a second waveguide, and afolded optical system with bi-directional, polarization-dependentfree-space paths that couple the first waveguide and the at least asecond waveguide. The optical system includes a birefringent pathseparator that is traversed by light propagating along the free-spacepaths in a first direction and in a second direction approximatelyopposite to the first direction.

Another embodiment of the invention is directed to an optical devicethat includes a first waveguide, a second waveguide coupled to the firstwaveguide via a first bi-directional, polarization dependent path, and athird waveguide coupled to the first waveguide via a secondbi-directional, polarization dependent path. A Wollaston prism isdisposed on the first and second bi-directional, polarization dependentpaths. The first and second bi-directional, polarization dependent pathsoverlap between the first waveguide and the Wollaston prism. A firstconverging optical subsystem is disposed to couple light between thesecond waveguide and the Wollaston prism and between the third waveguideand the Wollaston prism. The first converging optical subsystem includesat least one focusing element common to the first and the secondbi-directional, polarization dependent paths.

Another embodiment of the invention is directed to an optical devicethat includes a first waveguide, a second waveguide, a third waveguide,and a converging optical system. A birefringent optical system defines afirst polarized optical path between the first waveguide and the secondwaveguide and defines a second polarized optical path between the firstwaveguide and the third waveguide. The polarization of light propagatingalong the first polarized optical path is orthogonally polarized to thepolarization of light propagating along the second polarized opticalpath. The converging optical system includes at least one focusingelement disposed on both the first and second polarized optical pathswhere the first polarized optical path is spatially separated from thesecond polarized optical path.

Another embodiment of the invention is directed to an opticalcommunications system that includes a transmitting unit, a receivingunit and an optical transport system coupled to carry opticalinformation signals between the transmitting unit and the receivingunit. At least one of the transmitting unit, the receiving unit, and theoptical transport system include an optical device for coupling a firstlight beam to a second polarized light beam and a first beam to anorthogonally polarized light beam. The optical device includes a firstwaveguide and a second waveguide, and a birefringent optical system withbi-directional, polarization-dependent free-space paths. One of thepaths couples at least the first waveguide to the second waveguide. Thebirefringent optical system includes at least one prism for bending oneof the polarization-dependent paths in a clockwise direction and bendingone of the polarization-dependent paths in a counterclockwise direction.

Another embodiment of the invention is directed to an opticalcommunications system that includes a transmitting unit, a receivingunit and an optical transport system coupled to carry opticalinformation signals between the transmitting unit and the receivingunit. At least one of the transmitting unit, the receiving unit, and theoptical transport includes an optical device for coupling a first lightbeam to a second polarized light beam. The optical device includes afirst waveguide, a second waveguide and a folded optical system withbi-directional, polarization-dependent free-space paths that couple thefirst waveguide and at least the second waveguide. The folded opticalsystem includes a birefringent path separator that is traversed by lightpropagating along the free-space paths in a first direction and second,approximately opposite direction.

Another embodiment of the invention is directed to an opticalcommunications system that includes a transmitting unit, a receivingunit, and an optical transport system coupled to carry opticalinformation signals between the transmitting unit and the receivingunit. At least one of the transmitting unit, the receiving unit, and theoptical transport include an optical device for coupling a first lightbeam to a second polarized light beam. The optical device includes afirst waveguide, a second waveguide coupled to the first waveguide via afirst bi-directional, polarization dependent path, and a third waveguidecoupled to the first waveguide via a second bi-directional, polarizationdependent path. A Wollaston prism is disposed on the first and secondbi-directional, polarization dependent paths, the first and secondbi-directional, polarization dependent paths overlapping between thefirst waveguide and the Wollaston prism. A first converging opticalsubsystem couples light between the second waveguide and the Wollastonprism and between the third waveguide and the Wollaston prism. The firstconverging optical subsystem includes at least one focusing elementcommon to the first and the second bi-directional, polarizationdependent paths.

Another embodiment of the invention is directed to a method of couplinglight propagating in a first waveguide to polarized light propagating inat least a second waveguide. The method includes propagating the lightalong first and second bi-directional, polarization-dependent free-spacepaths. The polarization of light propagating along the firstbi-directional, polarization-dependent free-space path is orthogonal tothe polarization of light propagating along the second bi-directional,polarization-dependent free-space path. The method also includes bendingthe first polarization-dependent path in a counterclockwise directionand the second polarization-dependent path in a clockwise direction witha prism.

Another embodiment of the invention is directed to a method of couplinglight in a first waveguide to at least a second waveguide. The methodincludes propagating the light along bi-directional,polarization-dependent free-space paths. The paths include a first pathfor propagating polarized light and a second path for propagating lightpolarized orthogonally to polarization of light propagating along thefirst path. The method also includes traversing the light though abirefringent path separator in a first direction and in a second,approximately opposite direction.

Another embodiment of the invention is directed to a method of couplinglight between a first waveguide and second and third waveguides. Themethod includes propagating the light along bi-directional,polarization-dependent free-space paths. This includes propagatingpolarized light along a first path between the first and secondwaveguides and propagating polarized light, polarized orthogonallyrelative to light propagating along the first path, along a second pathbetween the first and third waveguides. The method also includesspatially separating and bending the first and second paths with aWollaston prism.

Another embodiment of the invention is directed to a method of couplingbetween a first waveguide and second and third waveguides. The methodincludes interacting the light with a birefringent optical system alonga first optical path between the first and second waveguides and asecond optical path between the first and third waveguides. Lightpropagating along the second path has a polarization orthogonal to apolarization of light propagating along the first path where the firstand second paths are spatially separated. The method also includescoupling the light between the birefringent optical system and thesecond and third waveguides with a converging optical subsystem havingat least one focusing optical element common to the first and secondpaths where the first and second paths are spatially separated.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates a polarization multiplexed opticalcommunications system.

FIG. 2 schematically illustrates an optical amplifier pumped bypolarized lasers.

FIG. 3 schematically illustrates a transmissive fiber optic polarizationseparator according to the prior art.

FIG. 4A schematically illustrates a reflective fiber optic polarizationseparator/combiner according to the present invention.

FIG. 4B schematically illustrates a transmissive fiber opticpolarization separator/combiner according to the present invention.

FIG. 5 schematically illustrates a transmissive fiber optic polarizationseparator/combiner that includes a birefringent material and a prism.

FIG. 6 schematically illustrates a transmissive-fiber optic polarizationseparator/combiner that includes a Wollaston prism.

FIG. 7 schematically illustrates a reflective fiber optic polarizationseparator/combiner that includes a birefringent material and apolarization rotator.

FIG. 8 schematically illustrates a reflective fiber optic polarizationseparator/combiner that includes a birefringent material and a facetedreflector.

FIG. 9 schematically illustrates a faceted reflector formed from twoprisms

FIG. 10 schematically illustrates a transmissive fiber opticpolarization separator incorporating a faceted birefringent beamseparator.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to optical fiber systems, and isbelieved to be particularly suited to combining and separating beams ofpolarized light. The approaches presented here may be simpler inconstruction, easier to align and lower in cost than conventionalsystems.

A typical fiber optic polarization combiner/separator is a three-portdevice that is designed to couple light propagating in a first waveguidewith a combination of polarization states to polarized light with afirst polarization direction propagating in a second waveguide andpolarized light with a second orthogonal polarization direction in athird waveguide. If the beams propagate from the second and thirdwaveguides to the first, the device acts as a polarization beamcombiner. If the beams propagate in the opposite direction from thefirst waveguide to the second and third waveguides, the device acts as apolarization separator.

Polarization combiner/separators may find a number of applications in anoptical fiber communications network. For example, thepolarization-multiplexed system 100 of FIG. 1 is designed to transportan optical signal with combined polarization from a transmitting unit105 to a receiving unit 110. The transmitting unit 105 includes twolaser transmitters 115, 120 that transmit polarized optical signals.These signals may, for example, include a number ofwavelength-multiplexed channels that are combined by the transmitters115, 120 according to a dense wavelength division multiplexing (DWDM)standard.

In the system of FIG. 1, the optical signal 125 from the lasertransmitter 115 may be polarized in a first direction and the opticalsignal 130 from the laser transmitter 120 may be polarized in a second,orthogonal direction. The optical signals 125, 130 from the lasertransmitters 115, 120 propagate through polarization-maintaining opticalfibers 135, 140 to the polarization combiner/separator 145. This unit isconfigured to combine the polarized signals 125, 130 to form an outputsignal 150 with combined polarization states. The output signal 150propagates through the transmitting unit output fiber 155 to thepolarization-maintaining optical transport system 160. The transportsystem 160 carries the polarization-multiplexed signal 150 to thereceiving unit. input fiber 165. In the receiving unit 110, a secondpolarization separator/combiner 170 separates the two polarizationcomponents of the optical signal 150 and couples the orthogonallypolarized optical signals 175, 180 to the optical fibers 185, 190. Theoptical receivers 195, 200 recover the information from the opticalsignals 175, 180.

Within an optical transport system, polarization separator/combiners maybe used, for example, to combine the pump and the signal beams in awaveguide amplifier. FIG. 2 schematically illustrates one particularembodiment of a waveguide amplifier unit 210 that increases the opticalpower of a polarized information signal 215. The information signal 215,which may have a wavelength of about 1.55 μm, is transported to theamplifier unit 210 by an optical fiber 220 and propagates from theamplifier unit 210 along the optical fiber 230. The fibers 220, 230 maybe polarization-maintaining.

The fiber amplifier 235, which may be an erbium fiber amplifier or othersuitable type of optical amplifier, is configured to transfer power fromthe pump beams 240, 245 obtained from one or more pump lasers, to theoptical signal 215. The polarization combiner 255 combines the polarizedsignal 215 propagating along the amplifier unit input fiber 220 with theorthogonally-polarized pump beam 240 that is transported to thepolarization combiner 255 by the optical fiber 250. The fiber amplifierinput fiber 260 transports the combined signal and pump beams to thefiber amplifier 235.

Optionally, a second pump beam 245 with mixed polarization states,propagating along optical fiber 270 may be coupled to the amplifieroutput fiber 275 by a WDM beam separator/combiner 280 thatcouples/separates beams according to wavelength. The WDM beamseparator/combiner 280 also couples the optical signal 215 to theamplifier unit output fiber 230. The pump beam 245 may include theoutput of two orthogonally-polarized pump beams 285, 290 that have beencombined, for example, by a polarization combiner 295. The optional pumpbeam 245 and the signal 215 propagate in opposite directions through thefiber amplifier 235 and the pump beam 240 and the signal 215 propagatein the same direction through the fiber amplifier 235.

A fiber optic polarization separator/combiner unit may include bulkoptical components and a plurality of focusing optical systems. Theoptical fibers that transport optical signals to and from a polarizationseparator/combiner unit are coupled to free space paths within the unitby focusing optical assemblies. The bulk optical components typicallyinteract with the light propagating along the free-space paths,separating and/or combining the light according to polarization. Thereis typically a one-to-one correspondence between focusing opticalassemblies and waveguides, the focusing optical assemblies beingpositioned to collimate the diverging light from input fibers and focusthe light propagating along the free space paths into the output fibers

A conventional fiber optic polarization separator, for example asillustrated in FIG. 3 as separator 300 typically includes three focusinglens assemblies 305–315 that couple the fibers 320–330 to the polarizingbeam splitter 335 along the free space beam paths 340–350. A combinedpolarization optical signal propagating along the waveguide 320 may becollimated by the focusing optical system 305 and directed to thepolarization beam splitter 335 along the optical path 340. Thepolarization beam splitter may, for example, be a cube polarizer. Thebeam splitter 335, directs light having a first polarization directionalong the optical path 350 and light polarized in the orthogonaldirection along the optical path 345. The focusing optical systems 310,315 focus the light propagating along the free space optical paths 345,350 into the output optical fibers 325, 330.

Conventional polarization separator/combiners share several commondisadvantages that derive from the one-to-one correspondence betweenfibers and focusing optical systems. In the polarization separator 300,for example, the low-loss propagation of light is facilitated by theaccurate alignment of the optical focusing assemblies 305–315 and theoptical fibers 320–330 and the alignment optical of the focusingassemblies 305–315 and the free space paths 340–350. Alignmenttolerances may be of the order of one micron and must be maintainedagainst both temperature variations and vibration during the operationallifetime of the polarization separator 300. Typically, the opticalcomponents are housed in a mechanical alignment and support assemblythat increases in complexity, size and cost with the number opticalcoupling components. It is, therefore, disadvantageous to use adedicated optical focusing assembly to couple each of the optical fibers320–325 to the free space paths 340–350.

According to the present invention, the number of optical focusingassemblies required to couple a reflective or transmissive bulk opticpolarization separator/combiner to a set of input/output fibers may bereduced through advantageous design of the bulk optic polarizationseparator/combiner and/or the coupling optical system. For example, FIG.4A schematically illustrates a reflective fiber optic polarizationseparator/combiner 410 that utilizes an optical coupling module 415 tocouple the optical fibers 420–430 to the free space optical paths435–445. The free space paths 435–445 are coupled according to thepolarization state of the light propagating along the free space pathsby the reflective free space optical system 450.

The optical coupling module 415 is further detailed in U.S. Pat. No.6,829,152, which is incorporated herein by reference. In one particularembodiment, an optical coupling module is an assembly that is couplable,for example, to the optical fibers 420–430, comprising a first focusingelement and a second focusing element, the first element positioned on afirst optical axis to receive output light beams from the optical fibersand direct the light beams to intersect an optical axis at a firstintersection position. The second focusing element is spaced apart fromthe first focusing element by a distance along the optical axis, theseparation being selected to parallelize the light beams received fromthe first optical element. When compared to conventional fiber couplingmethods, the coupling module method of FIG. 4A advantageously reducesthe number of focusing optical assemblies. Corollary advantages mayinclude a simplified alignment procedure and smaller transversedimensions of a packaged fiber optic polarization separator/combiner.

The number of focusing optical assemblies included in a transmissivefiber optic polarization separator/combiner may also be reduced bymodifying the bulk optical system in such a way that a single opticalfocusing assembly couples multiple optical fibers to their associatedfree space optical paths. For example, FIG. 4B schematically illustratesa transmissive fiber optic polarization separator 455 wherein thetransmissive free space optical system 460 interacts with combinedpolarization light propagating along the free space optical path 497from the fiber 495, dividing the light into two polarized light beamsthat are directed along the non-parallel free space optical paths 465,470. The free space optical paths 465, 470 are coupled to the opticalfibers 475, 480 by a first optical focusing assembly 485. A secondoptical focusing assembly 493 couples the optical fiber 495 to the freespace optical path 497.

The transmissive fiber optic polarization separator 455 illustrated inFIG. 4B has fewer optical focusing assemblies than conventionalpolarization separators. Corollary advantages of the invention mayinclude a simplified alignment procedure and smaller transversedimensions of a packaged fiber optic polarization separator/combiner.

FIG. 5 illustrates a transmissive fiber-optic polarization separator 500embodying features of the present invention. A light signal havingcombined polarization states is transported to the polarizationseparator 500 by an optical fiber 505. The divergent light beam 510exiting the fiber is approximately collimated by the first opticalfocusing assembly 515 that may be separated from the fiber end 520 by adistance, f₁, that is approximately equal to the focal length of theoptical focusing assembly 515. The first optical focusing assembly 515may be, for example, a single lens, or a combination of lenses. Thecollimated light beam from the optical focusing assembly 515 propagatesalong the free-space optical path 530 and interacts with thebirefringent beam separator 535. The birefringent beam separator 535may, for example, be a birefringent crystal with its optical axisdirection 540 oriented at an acute angle, α₁, to the light propagationdirection 545 of the free space path 530 in the plane of FIG. 5.

Within the birefringent beam separator 535, the portion of the lightthat is polarized in the ordinary direction propagates along a firstpath 547 and experiences the ordinary index of refraction, n_(o), whilethe portion of the light polarized in the extraordinary directionexperiences the extraordinary index of refraction, n_(e), and propagatesalong a second path 550 at an angle with respect to the first path 547.Thus, portions of a light beam with combined polarization states thatpropagates towards the birefringent beam separator 535 along the opticalpath 530 may be separated into two polarized light beams that propagatethrough the birefringent beam separator 535 along separate optical paths547, 550.

The physical separation of the beams at the surface 538 typicallyincreases with the absolute value of the difference between the ordinaryand extraordinary indices of refraction, also known as thebirefringence, and the length of the separator 535. The separator lengthmay be advantageously minimized by selecting a material with a largebirefringence at the desired operating wavelength. The separator 535 mayalso be advantageously selected to have high transparency at thewavelength of interest, physical properties that are insensitive totemperature and humidity, and physical properties that facilitateoptical polishing and coating. Materials that combine these propertiesat wavelengths between 1.5 μm and 1.65 μm include yttrium vanadate(YVO₄), rutile (TiO₂) and a-barium borate (α-BaB₂O₄).

The polarized light beams leaving the birefringent crystal 535 propagatealong free-space optical paths 555, 560 that are typically parallel andnon-overlapping. A prism 565, that may be a symmetric roof prism, withtwo facets tilted at acute angles γ, relative to the entrance surface578, bends the free space optical paths 555, 560 in clockwise andcounterclockwise directions so that the free space optical paths 555,560 intersect a plane containing the prism axis of symmetry 580 in aregion 583 located between the prism and the focusing optical assembly590. In some embodiments, the optical paths 555, 560 may also intersecteach other in the region 583.

A focusing optical assembly 590, which may comprise a single lens, or acombination of lenses, couples the optical paths 555, 560 to the opticalfibers 593, 595. Typically, the separation, f₂, of the optical fibers593, 595 from the focusing optical assembly 590 is approximately equalto the focal length of the focusing optical assembly 590. The distance,L, between the focusing optical assembly 590 and the region 583 istypically greater than or equal to the focal length of the focusingoptical assembly 590 and may be advantageously chosen to be equal to thedistance, f₂.

It may also be advantageous to join the birefringent beam separator 535and the prism 565 by decreasing the distance, d, between the two beamseparator 535 and the prism 565. Optical contacting techniques, forexample, may be used to join the two elements 535 and 565 and anantireflection (AR) coating may be applied to at least one of thesurfaces, 538, 578 to minimize the reflected portions of light beamspropagating along the paths 547, 550, 555 and 560. Alternatively, thesurfaces may be joined with an adhesive, for example, anultraviolet-light-cured transparent optical epoxy. The adhesive may beapplied to directly to the surfaces 538, 578 or to the edges of thebirefringent beam separator 535 and the prism 565 that are adjacent tothe surfaces 538, 578. Reflection at the prism facets 570, 575 may alsobe minimized by applying AR coatings to the facets.

In the illustrated embodiment 500, the first focusing optical assembly515 and the second optical focusing assembly 590 may be Geltech 350140lenses with a common focal distance, f=f₁=f₂. The prism may be apentagon that is formed from K10 glass that is supplied by SchottOptical Glass Co. with acute angles γ that are equal to 9.9°. Thebirefringent beam separator may be manufactured from yttrium vanadate.

The transmissive fiber optic polarization beam separator 500 may also beoperated as a transmissive fiber optic polarization beam combiner byreversing the direction of light propagation through the device.Polarized light beams propagating towards the optical focusing assembly590 along the optical fibers 593, 595 may be combined by thepolarization beam separator 500 to exit as a light beam with combinedpolarization states propagating away from the beam separator 500 alongthe optical fiber 505.

The transmissive fiber optic polarization beam separator 500 may also beused to couple counterpropagating beams. For example, a polarized beampropagating towards the polarization beam separator 500 along theoptical fiber 595 may be coupled to the fiber 505 as a beam thatpropagates from the beam separator 500. Simultaneously, anorthogonally-polarized beam propagating towards the beam separator 500along the fiber 505 may be coupled to the fiber 593 as a beampropagating away from the beam separator 500. Alternatively, a beam withmixed polarization propagating towards the device along the fiber 505may be separated into two orthogonally-polarized beams while polarizedbeams propagating towards the beam separator 500 along the fibers 593,595 may be combined into a mixed polarization beam propagating away fromthe beam separator along the fiber 505.

FIG. 10 illustrates another embodiment of a transmissive fiber-opticpolarization separator 1000. A light signal having combined polarizationstates is transported to the fiber optic polarization separator 1000 byan optical fiber 1005. The divergent light beam 1010 exiting the fiberis approximately collimated by the first optical focusing assembly 1015that may be separated from the fiber end 1020 by a distance, f₆, that isapproximately equal to the focal length of the optical focusing assembly1015. The optical focusing assembly 1015 may be, for example, a singlelens, or a combination of lenses. The collimated light beam from theoptical focusing assembly 1015 propagates along the free-space opticalpath 1030 and interacts with the faceted birefringent beam separator1020.

The faceted beam separator 1020 is formed from a birefringent materialand oriented so that light with a combined polarization statepropagating towards the birefringent beam separator 1020 is selectedaccording to polarization state at the surface 1035. For example, lightthat is polarized in the ordinary direction propagates along opticalpath 1050 and light polarized in the extraordinary direction propagatesalong the optical path 1045. Light propagating along the optical path1045 is coupled to the free space optical path 1065 at the facet 1055.The facet 1055 may be AR-coated to reduce reflection losses and istilted at an angle, δ₃, with respect to the input surface 1035. Theangle, δ₃, is selected to bend the light propagating along the path 1045in a clockwise direction. Light propagating along the optical path 1050is coupled to the free space optical path 1070 at the facet 1060. Thefacet 1060 is tilted at an angle, δ₄, with respect to the input surface1035. The angle, δ₄, is selected to bend the light propagating along thepath 1050 in a counterclockwise direction. The facet 1060 may also beAR-coated to minimize reflections.

The physical separation of the beams at the facets 1060, 1055 increaseswith the absolute value of the birefringence, and the length of theseparator 1020. The separator length may be advantageously minimized byselecting a material with a large birefringence at the desired operatingwavelength. The separator 1020 may also be advantageously selected tohave high transparency at the wavelength of interest, optical propertiesthat are insensitive to temperature and humidity, and physicalproperties that facilitate optical polishing and coating. Materials thatcombine these properties at wavelengths near 1.5 μm include yttriumvanadate (YVO₄), rutile (TiO₂) and alpha barium borate (α-BaB₂O₄).

A focusing optical assembly 1080 with an axis 1095 couples the freespace paths 1065, 1070 to the optical fibers 1085, 1090. The focusingoptical assembly may comprise, for example, a single lens or group oflenses. Typically, the free-space paths 1065, 1070 cross a planecontaining the axis 1095 in a beam crossing region 1075. They may alsointersect each other in the beam crossing region 1075. The opticalfibers 1085, 1090 may be disposed in a parallel configuration andseparated from the optical assembly 1080 by a distance, f₇, that may beequal to the focal length of the focusing optical assembly 1080.Advantageously, the distance, X, between focusing assembly 1015 alongthe optical fiber 1005 the optical assembly 1080 and the beam crossingregion 1075 may be approximately equal to the distance, f₇.

The fiber optical polarization separator 1000 may be operated as a fiberoptic polarization combiner by reversing the beam direction.Orthogonally-polarized light beams propagating towards the opticalfocusing assembly 1080 along the optical fibers 1085, 1090 will becombined by the polarization separator 1000 and propagate away from theoptical focusing assembly 1015 along the optical fiber 1005.Counterpropagating beams may also be combined and separatedsimultaneously.

FIG. 6 schematically illustrates another embodiment of a transmissivefiber optic beam separator 600 according to the present invention. Lightwith combined polarization states may be separated into polarized lightbeams by propagating light with combined polarization states along theoptical fiber 605 from left to right. The optical fiber 605 is coupledto the free space optical path 610 by the focusing optical assembly 615,that may comprise one or more lenses. Typically, the focusing opticalassembly 615 is separated from the end of the optical fiber 605 by adistance that is approximately equal to the focal length, f₃, of theoptical focusing assembly 615.

Light with combined polarization states propagating along the free-spaceoptical path 610 interacts with a Wollaston prism 625 and is separatedinto orthogonally-polarized beams that are bent and coupled to the freespace optical paths 635, 640. The Wollaston prism 625 may be formed in aconventional fashion from two birefringent prisms having a common prismangle, β₁. The center of the Wollaston prism 625 is separated by adistance, d₂, from the focusing optical assembly, 615.

The optical focusing assembly 645 couples the free space paths 635, 640to the optical fibers 650, 655 that are typically separated from theoptical focusing assembly 645 by a distance, f₄, that is approximatelyequal to the focal length of the optical focusing assembly 645. Theoptical focusing assembly 645 may also be positioned at a distance, d₂,from the Wollaston prism 625. Advantageously, the distances f₃ and f₄,and the distances, d₂ and d₃, may be equal to a common focal length, f₅.

The Wollaston prism 625 may advantageously be fabricated from yttriumvanadate or rutile. For example, the focusing optical assemblies 615,645 may be Geltech lenses with part number 350140 and the Wollastonprism may be formed from yttrium vanadate with a prism angle, β₁, of22.6°. Locating the components at distances, d₂ and d₃, that are equalto the common focal length of the Geltech lenses provides a physicalseparation distance, d₅, of the beams at the end of the polarized fibers655, 650 that is equal to 250 μm.

The transmissive beam separator/combiner 600 may also be operated as abeam combiner. In this case, polarized light beams propagating towardsthe left in the optical fibers 650, 655 are combined into a single beamthat is coupled into the optical fiber 605.

An embodiment of a reflective fiber optic polarization beamseparator/combiner 700 according to the present invention is illustratedschematically in FIG. 7. When operated as a beam separator, light withcombined polarization states propagates through the optical fiber 705 tothe beam separator/combiner 700. A coupling module 710 as described inU.S. patent application Ser. No. 09/181,145 couples the optical fiber705 to the free space optical path 715. Light propagating along theoptical path 715 is approximately collimated and interacts with thebirefringent beam separator 720. The beam separator may, for example, beformed from a birefringent material that is oriented with its opticalaxis direction 722 oriented at an acute angle, α₂, with respect to thedirection 726 of the free space optical path 715.

Light with ordinary polarization propagates through the birefringentbeam separator 720 in a first direction along the optical path 728.Light polarized in the extraordinary direction propagates through thebirefringent beam separator 720 in a second direction along the opticalpath 730. At the beam separator surface 735 the optical paths 728, 730are coupled to approximately parallel and separate bidirectional opticalpaths 738, 740.

The polarized light beams propagating from the birefringent beamsplitter 720 along the bidirectional optical paths 738, 740 interactwith a polarization rotator 745 and are redirected to the left by thereflector 750. As the polarized light beams propagate to the left fromthe reflector 750 to the birefringent beam separator 720 along thebidirectional optical paths 738, 740, they interact a second time withthe polarization rotator 745. The polarization rotator and mirror areconfigured to rotate the polarization of the light beams propagatingalong the bidirectional paths 738, 740 by approximately 90° as theytravel from the beam separator surface 735 to the reflector 750 andreturn to the beam separator surface 735. The polarization separator maybe, for example, a quarter wave retardation plate with its optical axistilted at 45° with respect to the polarization directions of the lightbeams propagating along the free space paths 738, 740 at the beamseparator surface 735.

Light propagating towards the beam separator 720 along the beam path 738travels through the beam separator 720 along the ordinary polarizationbeam path 755 and is coupled to the free space beam path 742. Lightpropagating towards the beam separator 720 along the beam path 740travels through the beam separator 720 along the extraordinarypolarization beam path 760 and is coupled to the beam path 745. Theoptical coupling module 710 couples the free space paths 742, 745 to theoptical fibers 765, 770.

The fiber optical polarization separator 700 may also operate as a beamcombiner if orthogonally polarized light beams propagate towards theseparator 700 along the optical fibers 755, 760. In this case, a beamwith combined polarization is transported away from the polarizationseparator 700 by the optical fiber 705. Simultaneous beam separation andcombination is also possible with bidirectionally propagating beams.

The mechanical complexity of the polarization separator 700 may beadvantageously decreased by combining the reflector 750 and polarizationrotator 745 into a single unit. This may be accomplished, for example,by joining the reflector 750 and polarization rotator 745 or by coatingthe reflector 750 directly on the surface 775 of the polarizationrotator 745. The mechanical complexity of the polarization separator 700may be also be decreased by joining the polarization rotator 745 and thebirefringent beam separator 720. Additional mechanical simplificationmay be accomplished either by coating the reflector 750 directly on thejoined polarization rotator 745 and beam separator 720 or byadditionally joining the polarization rotator 745, beam separator 720and reflector 750 to form a single mechanical assembly.

FIG. 8 illustrates another embodiment of a reflective fiber optic beamseparator/coupler 800 according to the invention. In polarizationseparating operation, light that may have a combined polarization statepropagates towards the separator/coupler 800 along the optical fiber805. Orthogonally-polarized light beams propagate to the left along theoptical fibers 810, 815. The coupling module 820 couples the opticalfiber 805 to the free space path 835, the optical fiber 810 to the freespace path 830 and the optical fiber 815 to the free space path 825. Theoperation and design of the coupling module is described in U.S. patentapplication Ser. No. 09/181,145.

Light with combined polarization states from the fiber 805 propagates tothe right along the free space path 835 and interacts with thebirefringent beam separator 840. The birefringent beam separator 840 maybe formed from a birefringent crystal with large birefringence and hightransparency at the wavelength of the light propagating through thedevice.

Light propagating along the free space path 835 propagates through thebirefringent beam selector along one of two optical paths according tothe polarization state of the light. Light polarized in theextraordinary direction propagates along the optical path 855 and lightpolarized in the ordinary direction propagates along the optical path860. At the beam separator surface 862, the optical path 855 is coupledto the free space optical path 865 and the optical path 860 is coupledto the free space optical path 870. Typically, the length of thebirefringent beam separator 840 is chosen to completely separate theoptical paths 855 and 860 at the surface 862.

Light propagating along the free space optical paths 865, 870 isredirected towards the birefringent beam separator 840 by the facetedreflector 875 that may have an axis of symmetry 878. Light propagatingto the right along the optical path 870 is redirected to the left alongoptical path 882 and light propagating to the right along the opticalpath 865 is redirected to the left along the optical path 880. Thefaceted reflector is typically positioned to symmetrically dispose thebeam path 870 and the beam path 882 on opposite sides of the symmetryaxis 878 and to symmetrically dispose the beam path 865 and the beampath 880 on opposite sides of the symmetry axis 878.

The birefringent beam separator extraordinary polarization optical path885 couples the free space path 880 to the free space 830 and theordinary polarization optical path 887 couples the free space path 882to the free space path 825. The optical coupling module couples the freespace paths 825, 830 and the optical fibers 810, 815 in such a way thethat light polarized in a first direction and propagating to the leftalong the optical path 830 is transported from the polarizationseparator 800 by the optical fiber 810 and orthogonally-polarized lightpropagating along the free space path 835 is transported from thepolarization separator 800 by the optical fiber 805.

The fiber optic polarization separator 800 may be operated as a fiberoptic polarization combiner by reversing the direction of lightpropagation. Orthogonally-polarized beams propagating towards thecoupling module 820 along the optical fibers 805, 810 may be combined bythe polarization separator 800 into a beam with combined polarizationstates propagating away from the optical coupler 820 on the opticalfiber 815. Simultaneous combining and separating operation may also bepossible with counterpropagating beams.

A faceted reflector may alternatively be an asymmetric assembly that isformed, for example, from two right angle prisms 905, 910. The facetedreflector assembly 900 that is illustrated in FIG. 9, for examplecomprises two right angle prisms that are optically coupled along theplane 910. Light propagating towards the prism 905 along the input freespace optical path 915 is coupled by the path 925 to light propagatingaway from the prism 910 along the output free space optical path 940.Light propagating towards the prism 905 along the input optical path 920is similarly coupled to the light propagating away from the prism 910along the optical path 935 by the optical path 930. While the facetedreflector lacks a symmetry axis, the beam paths are symmetric withrespect to the axis 950. For example, the beam paths 915 and 940 aresymmetrically disposed on either side of the axis 950 and the beam paths920 and 935 are symmetrically disposed on either side of the axis 950.The separation between symmetrically disposed beam paths may be adjustedby changing the displacement, k, between the prism surfaces.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. An optical device, comprising: a first waveguide and at least asecond waveguide at a first end of the device; a coupling modulecomprising at least a first lens; and a folded optical system withbi-directional, polarization-dependent free-space paths that couple thefirst waveguide and the at least a second waveguide, the folded opticalsystem including a birefringent path separator and a reflecting element,an optical path for unpolarized light from the first waveguide passingthrough the birefringent path separator where the unpolarized light isseparated into two orthogonal polarization components, the separatedorthogonal polarization components propagating along separated polarizedoptical pathhs to the reflecting element and being reflected by thereflecting element back through the birefringent path separator, onlyone of the polarized optical paths leading to the second waveguide;wherein light passing between one of the first waveguide and the atleast a second waveguide to the folded optical system passes through thefirst lens and light passing from the folded optical system to the otherof the first waveguide and the at least a second waveguide passes troughthe first lens.
 2. The optical device as recited in claim 1, whereinlight propagating in the second waveguide is polarized.
 3. The opticaldevice as recited in claim 1, wherein the light propagates through thefolded optical system from the first waveguide to the second waveguide.4. The optical device as recited in claim 1, wherein the lightpropagates through the folded optical system from the second waveguideto the first waveguide.
 5. The optical device as recited in claim 1,wherein the folded optical system includes an optical subsystem thatcouples light propagating along the free-space paths to the firstwaveguide and the second waveguide.
 6. The optical device as recited inclaim 5, wherein the optical subsystem is a coupling module.
 7. Theoptical device as recited in claim 1, wherein the birefringent pathseparator is formed from a crystal.
 8. The optical device as recited inclaim 1, wherein the folded optical system includes a polarizationrotator and the reflecting element comprises a mirror, the mirror beingdisposed to direct light propagating in a first direction to a second,reverse direction and the polarization rotator disposed to interact withthe light propagating along the free-space paths in both the first andsecond directions, rotating polarization of the light propagating in thesecond direction by approximately 90° relative to the light propagatingin the first direction.
 9. The optical device as recited in claim 8,wherein the mirror is formed on the polarization rotator.
 10. Theoptical device as recited in claim 8, wherein the polarization rotatoris attached to the birefringent path separator.
 11. The optical deviceas recited in claim 1, wherein the reflecting element includes a facetedreflector, a symmetry axis of the faceted reflector intersecting thebirefringent path separator and the faceted reflector, the symmetry axisbeing approximately parallel and equidistant from the light propagatingin the first direction and the light propagating in the second directionalong at least one of the free space paths.
 12. The optical device ofclaim 1, wherein the reflecting element comprises a mirror.
 13. Theoptical device of claim 1, wherein the reflecting element comprises aprism retroreflector arrangement.
 14. A device as recited in claim 1,wherein the at least a second waveguide comprises a third waveguidearranged so that light that propagates between the third waveguide andthe folded optical system passes through the first lens, a first of theorthogonal polarization components being associated with the secondwaveguide and a second of the orthogonal polarization components beingassociated with the third waveguide.